(1) Field of the Invention
This invention relates to a power transmission for a shuttle type vehicle having a plurality of hydraulically and selectively engaged clutches for the changing of direction and speed range and, more particularly, to a control for transmission clutches which, when a direction change is made, causes a higher speed range clutch to automatically and momentarily engage before the operator selected lower speed range clutch engages.
(2) Description of the Related Art
Powershift transmissions in which a torque convertor couples the transmission to the engine and the speed range and direction are controlled by the operator's selective engagement of a plurality of friction clutches are well known in the art. Since the transmission gears are continuously engaged it is not necessary for the vehicle to stop before the operator selects a change in direction. Power reversal, shifting the transmission direction control while the vehicle is moving, is a common feature of the operation of shuttle type vehicles, such as fork lift trucks. In a power reversal, the energy of the moving vehicle is absorbed in the drive train primarily as heat, due to slippage, in the engaging clutch. Powershift transmissions commonly include a means to control the rate of clutch engagement to permit the clutch to slip; smoothing the deceleration of the vehicle and limiting the torque developed in the drive train during a power reversal.
For lighter vehicles and slower speeds, slippage of a single clutch is usually adequate to absorb the energy of the moving vehicle. However, with heavier vehicles, higher speeds and greater frequency of power reversal larger, more expensive clutches are required to withstand the thermal load of power reversals. The higher gear ratios, common in the lower speed ranges of heavy vehicles, also make it difficult to control clutch engagement to avoid excessive drive train torque during a power reversal while maintaining a short delay time for smooth transmission performance.